Implements XPath functions -- each function is derived from Function,
FunctionOneArg, Function2Args, etc, with number-of-arguments checking being
applied mainly at compile time -- this package only implements XPath functions,
XSLT functions are found in the "templates" package.

XLST is a stylesheet language for transforming XML documents into other XML documents, HTML documents,
other document types, or text. The language includes the XSL Transformation vocabulary and XPath, a language for
addressing parts of an XML document. An XSL stylesheet describes how to transform the tree of nodes in the XML
input into another tree of nodes.